What was done
Working with the ERSST (Smith and Reynolds, 2003), HadISST1 (Rayner et al., 2003), and GISST2.3b (Rayner et al., 1996) global sea surface temperature (SST) data sets, the authors reconstructed time series of an important sea-surface thermal stressor - degree heating months (DHMs) - for the time period 1871-2002 at grid sites around the world, analyzing the results in considerable detail for several locations of notable coral cover, including the Caribbean, Northwest Hawaiian Islands (NWHI), and parts of other regions such as the Great Barrier Reef (GBR).

What was learned
Barton and Casey report that "while coral bleaching and anomalously warm SSTs have occurred over much of the world in recent decades, case studies in the Caribbean, NWHI, and parts of other regions such as the GBR, exhibited SST conditions and cumulative thermal stress prior to 1979 comparable to those conditions observed during the strong, frequent coral bleaching events since 1979." In fact, they found that warm-season mean SST anomalies and DHM values for Sombrero Reef and Discovery Bay in the Caribbean were often as warm in the 1940s-1960s as they were in the 1979-2002 period, "with notable peaks in 1944, 1949, 1957-1958, 1962 and 1968 on Sombrero Reef and 1952, 1958, 1962-1963, 1966 and 1969 in Discovery Bay." What is more, they say that "all three reconstructions show 1957-1966 being the decade of maximum mean DHM for Sombrero Reef," while "for Discovery Bay, the decade of maximum mean DHM for ERSST is 1951-1960 and 1960-1969 for HadISST1 and GISST2.3b."

With respect to the NWHI locations of Midway Island and Pearl and Hermes Reef, Barton and Casey found that these sites also "experienced warm SST anomalies of a similar magnitude during the 1960s, with 1968 being the warmest," and they found that "Midway Island and Pearl and Hermes Reef show their decade of maximum mean DHM to be 1959-1968 for ERSST," although it is 1993-2002 for the HadISST1 and GISST2.3b data sets. In addition, the two researchers determined that in all three reconstructions, "the northern Great Barrier Reef shows [the] decade of maximum mean DHM values occurring in the 1960s."

What it means
In the words of Barton and Casey, the results of their study "show that SST conditions similar to post-1979 levels occurred in the Caribbean, NWHI, and at least part of the GBR before the advent of large-scale coral bleaching," and they therefore wonder "why there is no record of coral bleaching in the Caribbean, NWHI, and GBR before 1979," concluding that "it seems climate change-driven changes to SST alone may not explain the increased incidence of large-scale bleaching in the Caribbean and NWHI, and perhaps in segments of the GBR." Citing the concurrent buildups of other coral reef stressors, including increased instances of disease, increased pollution, increased sedimentation, over-fishing, mechanical destruction of corals, "and possibly others," they conclude "there is evidence supporting long-term changes in environmental factors that could potentially act synergistically with elevated SSTs to bring about coral bleaching in the recent era while explaining its absence during the 1940s-1960s," and that "it appears as though climate warming alone may not explain the increased frequency of bleaching [after 1979]."

References
Rayner, N.A., Horton, E.B., Parker, D.E., Folland, C.K. and Hackett, R.B. 1996. Version 2.2 of the Global sea-ice and sea surface temperature data set, 1903-1994. Climate Research Technical Note 74 (CRTN74), Hadley Centre for Climate Prediction and Research, Meteorological Office, London, Road, Bracknell, Berkshire RG12 2SY.

Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C. and Kaplan, A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: No. D14, 4407.

Smith, T.M. and Reynolds, R.W. 2003. Extended reconstruction of global sea surface temperatures based on COADS data (1854-1997). Journal of Climate 16: 1495-1510.